In OVCAR3 cells, IFNAR1/IFNAR2-RNAi (80-90% knock down) or IRF9-RNAi (80% knockdown) completely inhibited the antiproliferative activity of IFN-&#945;. On the other hand, the IRF9-RNAi demonstrated no inhibition of the antiproliferative activity of IFN-&#947;. These results suggest that IFN-&#945; signals through IFNAR1/IFNAR2 and utilize IRF9 to elicit the antiproliferative activity in OVCAR3 cells. Conversely, Stat1-RNAi (70%knockdown) demonstrated no inhibition of the antiproliferative activity of IFN-&#945;, but partial inhibition of that of IFN-&#947;, suggesting that a decrease in constitutive Stat1 protein expression (&#8804; 70% reduction) have no influence directly on the antiproliferative activity of IFN-&#945;. Furthermore, TRAIL gene expression (related to apoptosis) was inhibited by IFNAR1-RNAi or IRF9-RNAi, but not by Stat1-RNAi following IFN-&#945; treatment, suggesting that induction of TRAIL gene plays a role in eliciting the antiproliferative activity of IFN-&#945;. In addition, the microarray analysis also revealed that IRF9-RNAi demonstrated no significant inhibition of Stat1 gene expression in response to IFN-&#945;2c. Collectively, these findings suggest that neither up-regulation of Stat1 gene expression nor down-regulation of Stat1 protein expression affects the antiproliferative activity of IFN-&#945; in OVCAR3 cells. For further investigation of the role of IRF9 in the antiproliferative activities of IFNs, stable transfectants of the IRF9 gene were constructed using pUNO plasmids in A549 (human lung carcinoma) and T98G (human glioblastoma multiforme) cells that were resistant to IFN-&#945; and then antiproliferative activity was determined on day 5. In these cells, the IRF9 overexpression showed dose-dependent enhancement of the antiproliferative activity of IFN-&#945;, but not of IFN-&#947;. Thus, IRF9 may have a predominant role in the antiproliferative activity of IFN-&#945; in the JAK-STAT pathway. However, other factors downstream of the JAK-STAT signaling, such as TRAIL, may also be essential for the antiproliferative activities of IFNs. [unreadable] [unreadable] Assays performed to explore how the combination of human monocytes and IFN and their effect on tumor cells showed that IFN-treated monocytes exhibit an enhanced antitumor effect on human osteosarcoma cells and that cell contact is important for this effect. It was also observed that there was enhanced monocyte antitumor activity with a combination of Type I and II IFNs.